The launch originated from North Pyongan province, the agency said, citing the South's Joint Chiefs of Staff.

Earlier this month, the nuclear-armed North carried out similar launches on two occasions -- Pyongyang said they had conducted "long-range artillery" drills, but Japan said the projectiles appeared to be ballistic missiles.

Shortly before the Yonhap report about the launch, the North's official Korean Central News Agency (KCNA) reported that the country would convene its rubber-stamp parliament, the Supreme People's Assembly (SPA), on April 10.

The latest launch comes as a prolonged hiatus in disarmament talks with the United States drags on.

The North has been continuing to refine its weapons capabilities, analysts say, more than a year after a summit between its leader Kim Jong Un and US President Donald Trump broke down in Hanoi.

Pyongyang is under multiple sets of United Nations, US and other sanctions over its weapons programmes.

The North carried out a series of weapons trials late last year, the last of them in November, which it often described as multiple launch rocket systems, although others called them ballistic missiles.

It also conducted static engine tests, most recently in December.

Pyongyang set Washington a unilateral deadline of the end of 2019 to offer fresh concessions on sanctions relief, and at a party meeting in late December, Kim declared the North no longer considered itself bound by its moratoriums on nuclear and intercontinental ballistic missile tests.

Heightened tensions in 2017 were followed by two years of nuclear diplomacy between Pyongyang and Washington, including three meetings between Kim and Trump, but little tangible progress was made.